US20060072112A1 - Method and system for spectral stitching of tunable semiconductor sources - Google Patents
Method and system for spectral stitching of tunable semiconductor sources Download PDFInfo
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Definitions
- a number of general configurations are used for tunable source spectroscopy systems.
- the lasers have advantages in that very intense tunable optical signals can be generated.
- a different configuration uses the combination of a broadband source and a tunable passband filter, which generates the narrowband signal that illuminates the sample.
- tunable lasers were based on solid state gain media. While often powerful, these systems typically have high power consumptions. In contrast, tunable semiconductor laser systems have the advantage of relying on small, efficient, and robust semiconductor sources.
- SOAs semiconductor optical amplifiers
- MEMS microelectromechanical system
- Fabry-Perot tunable filters as described in U.S. Pat. No. 6,339,603, by Flanders, et al., which is incorporated herein by this reference in its entirety.
- the tunable filter is an acousto-optic tunable filter (AOTF) and the broadband signal is generated by a diode array or tungsten-halogen bulb, for example.
- AOTF acousto-optic tunable filter
- the broadband signal is generated by a diode array or tungsten-halogen bulb, for example.
- SLEDs superluminescent light emitting diodes
- MEMS Fabry-Perot tunable filter to generate the tunable optical signal. See U.S. patent appl. Ser. No. 10/688,690, filed on Oct. 17, 2003, by Atia, et al., which is incorporated herein by this reference in its entirety.
- the MEMS device is highly mechanically and spectrally stable and can handle high powers and can further be much smaller and more energy-efficient than typically large and expensive AOTFs. Moreover, the SLEDS can generate very intense broadband optical signals over large bandwidths, having a much greater spectral brightness than tungsten-halogen sources, for example.
- the present invention is directed to a multi semiconductor source tunable spectroscopy system. It has two or more semiconductor sources for generating tunable optical signals that are tunable over different spectral bands. The system enables the combination of these tunable signals to form an output signal that is tunable over a combined band including these individual spectral bands of the separate semiconductor sources.
- Critical to the invention is a system for compensating for spectral roll-off associated with the semiconductor sources. Specifically, near the limits of the semiconductor sources' spectral bands, the power in the tunable signal tends to degrade or decrease. The present invention compensates for this roll-off.
- the invention features a multi semiconductor source tunable spectroscopy system.
- This system comprises at least two semiconductor sources for generating tunable optical signals. These optical signals are tunable over different spectral bands, but combined into an output signal that is tunable over a combined scan band.
- a control system compensates for the spectral roll-off of the at least two semiconductor sources over the combined scan band. In this way, the system can produce an effectively wider scan band by combining the output from two semiconductor sources, without incurring penalities associated with available power in the tunable optical signals near the limits of the spectral bands of the separate semiconductor sources.
- control system comprises at least one optical signal regulator for controlling an amplitude of the output signal over the combined scan band of the system.
- control system comprises at least one source driver circuit for controlling an amplitude of the output signal over the scan band of the system by regulating the drive current to the semiconductor sources.
- control system comprises an output signal detector for detecting the output signal before interaction with a sample.
- a controller then corrects a sample signal, which is produced by interaction of the output signal with the sample, in response to spectral roll-off detected by the output signal detector.
- This controller can be implemented as a digital, analog, or hybrid digital/analog control circuit.
- the semiconductor sources are light emitting diodes, such as superluminescent light emitting diodes (SLED), generating broadband signals.
- the broadband signals are converted to the tunable signals by at least one tunable filter.
- the semiconductor sources are lasers comprising laser cavities.
- the semiconductor optical amplifier (SOA) chips are located within the cavities with tunable filters.
- the cavities can be linear or ring configurations.
- the diodes e.g., SLED or SOA
- the tunable filters are implemented on a bench.
- one diode and one filter are implemented in common on a single bench.
- all of the tunable filters and diodes are integrated together on a common bench.
- the tunable filter is a micro-electromechanical system (MEMS) Fabry-Perot tunable filter.
- MEMS micro-electromechanical system
- FIG. 1A is a perspective view of a tunable SLED source with a wavelength referencing system, to which the present invention is applicable;
- FIG. 1B is perspective view of a semiconductor tunable laser (linear cavity) source with a wavelength referencing system, to which the present invention is applicable;
- FIG. 1C is perspective view of a semiconductor tunable laser (ring cavity) source with a wavelength referencing system, to which the present invention is applicable
- FIG. 2 is a block diagram of a multi semiconductor tunable source spectroscopy system, to which the present invention is applicable;
- FIG. 3 shows the overlapping gain spectrums for various semiconductor chips and the mode or passband selection provided by the tunable filter
- FIG. 4 shows spectral stitching system for a semiconductor source spectroscopy system according to a first embodiment of the present invention, utilizing drive current feedback control of the semiconductor sources;
- FIG. 5 shows spectral stitching system for a semiconductor source spectroscopy system according to a second embodiment of the present invention, utilizing optical signal regulator control of the tunable optical signal
- FIG. 6 shows spectral stitching system for a semiconductor source according to a third embodiment of the present invention, utilizing software-based baselining of the tunable optical signal.
- FIGS. 1A through 1C illustrate various embodiments of the semiconductor sources that are preferably used in the multi semiconductor source tunable spectroscopy system according to the present invention.
- FIG. 1A shows a tunable superluminescent light emitting diode (SLED) semiconductor tunable source 200 .
- SLED superluminescent light emitting diode
- the tunable SLED 200 comprises a diode, specifically SLED, chip 210 .
- This SLED chip 210 is an edge-emitting device that is installed on a submount 212 that supports the SLED chip 210 over an optical bench 205 .
- the broadband optical signal produced by the SLED chip 210 is collimated by a lens optical component 216 .
- the lens optical component 216 includes a mounting structure 218 and a lens element substrate 220 , in which the optically curved surface of the lens is formed.
- this mounting structure 218 allows for post installation alignment of the lens element 220 with respect to the SLED chip 210 and the source's optical train, generally.
- the broadband signal from the SLED chip 210 is converted to a narrowband, tunable optical signal by a tunable filter 214 .
- the tunable filter 214 is a microelectrical mechanical system (MEMS) Fabry Perot tunable filter. Curved-flat or flat-flat Fabry-Perot cavities are preferably used.
- the Fabry-Perot tunable filter is manufactured as described in U.S. Pat. Nos. 6,608,711 or 6,373,632, which are incorporated herein by this reference.
- the tunable signal on exiting from the tunable filter 214 , is then coupled through an optional optical signal regulator 218 .
- this regulator controls the amplitude of the tunable optical signal.
- this tunable optical signal regulator 218 is an amplifier or other controlled gain element.
- the tunable optical signal regulator 218 is a tunable attenuator.
- it is a solid state, ceramic, high-speed optical attenuator.
- One commercial example is provided by Boston Applied Technologies, Inc. These are ceramic devices that are electrically modulated in order to control the level of the attenuation applied to the tunable signal.
- a beam splitter 220 diverts a portion of the signal to a source detector 222 .
- This comprises a detector chip 224 and a windowing element 226 that prevents a stray light from being received at the detector substrate 224 by ensuring that only columnated light reaches the detector substrate 224 via an optical port 226 .
- the light that is not diverted by the tap 220 is then focused by a second lens component 222 into an optical fiber 224 , which carries the light to the sample.
- the endface 226 of the optical fiber 224 is secured to the optical bench 205 by a fiber mounting structure 228 , which again allows for the post installation alignment of the optical fiber end-face in order to maximize the coupling efficiency of the optical train.
- FIG. 1B illustrates a second embodiment of the tunable semiconductor source 200 .
- the tunable semiconductor source 200 again comprises an optical bench 205 .
- a semiconductor optical amplifier (SOA) chip 240 is used as the semiconductor gain medium. This chip is also secured to the bench 205 via a submount 212 .
- SOA semiconductor optical amplifier
- the SOA chip 240 is a reflective SOA.
- the SOA chip's back facet 242 has a reflective coating in order to function as a back reflector of the laser cavity of the semiconductor source 200 .
- Light exiting from the SOA chip 240 is columnated by a first lens component 216 . It passes through an isolator at 244 to a second lens component 246 and then to a tunable filter 214 .
- the filter 214 functions to control which of the longitudinal optical nodes have a net gain in the laser cavity of the laser tunable semiconductor tunable source 200 .
- Light exiting from the filter 214 is collimated by a third lens component 248 .
- more or less isolators 244 are used in the laser cavity.
- the tunable laser source has no isolators.
- the Fabry Perot tunable filter 214 is angled with respect to the optical axis of the laser cavity in order to provide tilt isolation.
- the angling of the Fabry Perot tunable filter is relatively small between one and five degrees.
- additional isolators are used to prevent back reflections to the gain chip.
- Light passes through an optical signal regulator 218 , as described previously.
- the light then passes through a tap 220 that diverts part of the optical signal to the source detector 222 .
- the remaining signal is coupled into the optical fiber 224 via a fourth lens component 250 .
- the mirror 272 defining the end of the cavity is located in the fiber 224 . In other examples, the mirror is located between the regulator 218 and the filter 214 , thus placing the regulator 218 and the detector 222 outside of the laser cavity.
- FIG. 1C shows still another example of the semiconductor tunable source 200 .
- This is also a laser configuration. Instead of a linear cavity, however, it uses a ring laser cavity 270 .
- a ring cavity is defined by mirrors 272 , 274 , 276 , and 278 .
- An isolator 244 is located in the ring laser cavity in order to ensure that light travels in only one direction through the ring.
- Mirror 272 is partially reflective in order to provide a portion of the optical signal circulating in the ring cavity as a tunable output signal.
- a SOA chip 240 provides the gain in the laser cavity 270 .
- Tunable filter 214 again controls which longitudinal modes of the laser cavity have net gain.
- lens components 282 , 284 , 286 , and 288 provide coupling into and out of the tunable filter 214 and the SOA gain chip 240 .
- An optical signal regulator 218 such as a tunable attenuator, is provided at the output of the ring cavity 270 .
- a tap 220 is also provided in one embodiment to couple part of the tunable optical signal to the source detector 222 .
- a lens component 250 couples the remaining light into the optical fiber at 224 that conveys the light to the sample.
- FIG. 2 illustrates the use of the tunable semiconductor sources 200 illustrated in FIGS. 1A through 1C in the construction of a multi semiconductor tunable source spectroscopy system 100 .
- two semiconductor sources 200 - 1 , 200 - 2 (source 1 , source 2 ) produce tunable signals covering different spectral bands on optical paths or fibers 224 - 1 , 224 - 2 . These are combined to produce an output signal on path or fiber 311 via a combiner 310 .
- the combiner 310 is a fiber coupler.
- the combiner is implemented on an optical bench using polarization filters or a spectral, dichroic filters, for example.
- the output signal on fiber 311 has a combined scan band that combines the different spectral bands of the two semiconductor sources 200 - 2 , 200 - 2 . In this way, it has a combined scan band that enables scanning over a broader scan band than could be obtained from the sources 200 - 1 , 200 - 2 individually.
- this output signal on fiber 311 is conveyed by an output signal coupler 312 to an output signal detector 314 .
- the remaining output signal is conveyed to a sample 10 and then detected by a sample detector 12 .
- FIG. 3 illustrates the spectral roll-off that is characteristic of the semiconductor sources 200 .
- Overlaid onto the source spectra 410 - 1 , 410 - 2 are the tunable filters' passbands 412 - 1 and 412 - 2 for each of the sources 200 - 1 , 200 - 2 .
- the semiconductor chips potentially emit in corresponding bands 410 - 1 and 410 - 2 .
- the tunable filter passbands 412 - 1 , 412 - 2 will filter out or block the other regions of the spectrums 410 - 1 , 410 - 2 in the case of the tunable SLED sources ( FIG. 1A ) or constrain the source to only emit light generated by the optical modes within the passbands 412 - 1 , 412 - 2 in the case of the tunable laser sources ( FIGS. 1B and 1C ).
- the sources 200 - 1 , 200 - 2 cover different spectral bands.
- tunable source 200 - 1 covers band band- 1 stretching from approximately 1330 to 1470 nanometers (nm).
- source 2 200 - 2 covers band- 2 stretching from approximately 1470 to 1610 nanometers.
- the tunable signals varies in amplitude by approximately 20 decibels dB over this range due to the gain spectrums 410 - 1 and 410 - 2 associated with the separate sources 200 - 1 , 200 - 2 .
- these gain bands 410 - 1 , 410 - 2 suffer substantial roll-off near the limits of the associated semiconductor sources such that it causes changes in the amplitude of the resulting output signal which can impact the analysis of the spectral response of the sample 10 .
- FIG. 4 illustrates a first embodiment control system for compensating for the spectral roll-off of the at least two semiconductor sources, e.g., SLED sources 210 or SOA sources 240 , over the combined scan band provided by the combination of the tunable signals from the two semiconductor sources 200 - 1 , 200 - 1 .
- the at least two semiconductor sources e.g., SLED sources 210 or SOA sources 240
- source one 200 - 1 is driven by a source drive circuit of a control system 400 including a system detector 314 and a controller 412 .
- the source detectors 224 are used to sample the magnitude of the signals produced by the sources 200 - 1 , 200 - 2 .
- the source drive circuit 410 controls the current to the diode chips, e.g., SLED chips 210 of the tunable SLED of FIG. 1A , or SOA chips 240 of the tunable lasers of FIG. 1B, 1C .
- the modulation of the drive current maintains the levels of the respective tunable signals at a fixed level such as level 450 of FIG. 3 .
- the power or drive current to the chips 210 / 240 in the sources 200 - 1 , 200 - 2 is increased in order to achieve a stable level to the output signal across the entire combined scan band.
- FIG. 5 illustrates a second embodiment of the control system 400 .
- the control system comprises the attenuators or optical signal regulators 218 of the sources 200 - 1 , 200 - 2 .
- the system detector 314 or alternatively the source detectors 224 are used to sample the level of the output signal or the tunable signals from the separate sources at 200 - 1 , 200 - 2 .
- an attenuator drive circuit 414 of the system controller 400 is used to drive the attenuators 218 in order to attenuate the separate tunable optical signals in order to achieve a stable level 450 to the output signal across the entire combined scan band 510 .
- FIG. 6 shows still another embodiment to the control system.
- the control system 400 comprises the system detector 314 or alternatively the source detectors 224 .
- the signal from the detector is then used to modify the sampled signal detected by the sample detector 12 .
- the signal from the sample detector 12 is divided by the signal from the output signal detector 314 or source detectors 224 .
- an analog divider circuit in controller 400 is used in one embodiment.
- the controller 440 generates a compensated sample signal after accounting for the variation in the tunable output signal as detected by the detector 314 .
- this compensation is performed digitally. Specifically, analog to digital converters are used to sample the output from the output signal detector 314 and the sample detector 312 . The controller then operates a software program that performs the compensation.
Abstract
Description
- This application is a continuation in part of to application Ser. No. (Attorney docket 0005.1130US1, filed on Sep. 29, 2004, by Flanders, et al., which application is incorporated herein by this reference in its entirety.
- A number of general configurations are used for tunable source spectroscopy systems. The lasers have advantages in that very intense tunable optical signals can be generated. A different configuration uses the combination of a broadband source and a tunable passband filter, which generates the narrowband signal that illuminates the sample.
- Historically, most tunable lasers were based on solid state gain media. While often powerful, these systems typically have high power consumptions. In contrast, tunable semiconductor laser systems have the advantage of relying on small, efficient, and robust semiconductor sources. One configuration uses semiconductor optical amplifiers (SOAs) and microelectromechanical system (MEMS) Fabry-Perot tunable filters, as described in U.S. Pat. No. 6,339,603, by Flanders, et al., which is incorporated herein by this reference in its entirety.
- In commercial examples of the broadband source/tunable filter tunable source configuration, the tunable filter is an acousto-optic tunable filter (AOTF) and the broadband signal is generated by a diode array or tungsten-halogen bulb, for example. More recently, some of the present inventors have proposed a tunable source that combines superluminescent light emitting diodes (SLEDs) and a MEMS Fabry-Perot tunable filter to generate the tunable optical signal. See U.S. patent appl. Ser. No. 10/688,690, filed on Oct. 17, 2003, by Atia, et al., which is incorporated herein by this reference in its entirety. The MEMS device is highly mechanically and spectrally stable and can handle high powers and can further be much smaller and more energy-efficient than typically large and expensive AOTFs. Moreover, the SLEDS can generate very intense broadband optical signals over large bandwidths, having a much greater spectral brightness than tungsten-halogen sources, for example.
- One drawback associated with semiconductor spectroscopy systems, however, is limited spectral width. Often a single semiconductor chip is not available that can cover the entire desired scan band.
- As a result, a number of tunable semiconductor source have been proposed that are based on using the emission from multiple SLED's or SOA chips in the context of tunable SLED or tunable laser devices.
- The present invention is directed to a multi semiconductor source tunable spectroscopy system. It has two or more semiconductor sources for generating tunable optical signals that are tunable over different spectral bands. The system enables the combination of these tunable signals to form an output signal that is tunable over a combined band including these individual spectral bands of the separate semiconductor sources.
- Critical to the invention, however, is a system for compensating for spectral roll-off associated with the semiconductor sources. Specifically, near the limits of the semiconductor sources' spectral bands, the power in the tunable signal tends to degrade or decrease. The present invention compensates for this roll-off.
- In general, according to one aspect, the invention features a multi semiconductor source tunable spectroscopy system. This system comprises at least two semiconductor sources for generating tunable optical signals. These optical signals are tunable over different spectral bands, but combined into an output signal that is tunable over a combined scan band.
- A control system compensates for the spectral roll-off of the at least two semiconductor sources over the combined scan band. In this way, the system can produce an effectively wider scan band by combining the output from two semiconductor sources, without incurring penalities associated with available power in the tunable optical signals near the limits of the spectral bands of the separate semiconductor sources.
- In one embodiment, the control system comprises at least one optical signal regulator for controlling an amplitude of the output signal over the combined scan band of the system.
- In another embodiment, the control system comprises at least one source driver circuit for controlling an amplitude of the output signal over the scan band of the system by regulating the drive current to the semiconductor sources.
- In still another embodiment, the control system comprises an output signal detector for detecting the output signal before interaction with a sample. A controller then corrects a sample signal, which is produced by interaction of the output signal with the sample, in response to spectral roll-off detected by the output signal detector. This controller can be implemented as a digital, analog, or hybrid digital/analog control circuit.
- In one embodiment, the semiconductor sources are light emitting diodes, such as superluminescent light emitting diodes (SLED), generating broadband signals. The broadband signals are converted to the tunable signals by at least one tunable filter.
- In other embodiments, the semiconductor sources are lasers comprising laser cavities. The semiconductor optical amplifier (SOA) chips are located within the cavities with tunable filters. Depending on the implementation, the cavities can be linear or ring configurations.
- In the preferred embodiment, the diodes, e.g., SLED or SOA, and the tunable filters are implemented on a bench. In one embodiment, one diode and one filter are implemented in common on a single bench. In other embodiments, all of the tunable filters and diodes are integrated together on a common bench. In the preferred embodiment, the tunable filter is a micro-electromechanical system (MEMS) Fabry-Perot tunable filter.
- The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.
- In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings:
-
FIG. 1A is a perspective view of a tunable SLED source with a wavelength referencing system, to which the present invention is applicable; -
FIG. 1B is perspective view of a semiconductor tunable laser (linear cavity) source with a wavelength referencing system, to which the present invention is applicable; -
FIG. 1C is perspective view of a semiconductor tunable laser (ring cavity) source with a wavelength referencing system, to which the present invention is applicable -
FIG. 2 is a block diagram of a multi semiconductor tunable source spectroscopy system, to which the present invention is applicable; -
FIG. 3 shows the overlapping gain spectrums for various semiconductor chips and the mode or passband selection provided by the tunable filter; -
FIG. 4 shows spectral stitching system for a semiconductor source spectroscopy system according to a first embodiment of the present invention, utilizing drive current feedback control of the semiconductor sources; -
FIG. 5 shows spectral stitching system for a semiconductor source spectroscopy system according to a second embodiment of the present invention, utilizing optical signal regulator control of the tunable optical signal; and -
FIG. 6 shows spectral stitching system for a semiconductor source according to a third embodiment of the present invention, utilizing software-based baselining of the tunable optical signal. -
FIGS. 1A through 1C illustrate various embodiments of the semiconductor sources that are preferably used in the multi semiconductor source tunable spectroscopy system according to the present invention. - Specifically,
FIG. 1A shows a tunable superluminescent light emitting diode (SLED)semiconductor tunable source 200. - In more detail, the
tunable SLED 200 comprises a diode, specifically SLED,chip 210. ThisSLED chip 210 is an edge-emitting device that is installed on asubmount 212 that supports theSLED chip 210 over anoptical bench 205. - The broadband optical signal produced by the
SLED chip 210 is collimated by a lensoptical component 216. In the illustrated embodiment, the lensoptical component 216 includes a mountingstructure 218 and alens element substrate 220, in which the optically curved surface of the lens is formed. - The use of this mounting
structure 218 allows for post installation alignment of thelens element 220 with respect to theSLED chip 210 and the source's optical train, generally. - The broadband signal from the
SLED chip 210 is converted to a narrowband, tunable optical signal by atunable filter 214. In the present embodiment, thetunable filter 214 is a microelectrical mechanical system (MEMS) Fabry Perot tunable filter. Curved-flat or flat-flat Fabry-Perot cavities are preferably used. In one example, the Fabry-Perot tunable filter is manufactured as described in U.S. Pat. Nos. 6,608,711 or 6,373,632, which are incorporated herein by this reference. - The tunable signal, on exiting from the
tunable filter 214, is then coupled through an optionaloptical signal regulator 218. In the preferred embodiment, this regulator controls the amplitude of the tunable optical signal. - In one embodiment, this tunable
optical signal regulator 218 is an amplifier or other controlled gain element. In the present embodiment, the tunableoptical signal regulator 218 is a tunable attenuator. Preferably, it is a solid state, ceramic, high-speed optical attenuator. One commercial example is provided by Boston Applied Technologies, Inc. These are ceramic devices that are electrically modulated in order to control the level of the attenuation applied to the tunable signal. - A
beam splitter 220 diverts a portion of the signal to asource detector 222. This comprises adetector chip 224 and awindowing element 226 that prevents a stray light from being received at thedetector substrate 224 by ensuring that only columnated light reaches thedetector substrate 224 via anoptical port 226. - The light that is not diverted by the
tap 220 is then focused by asecond lens component 222 into anoptical fiber 224, which carries the light to the sample. In the preferred embodiment, theendface 226 of theoptical fiber 224 is secured to theoptical bench 205 by afiber mounting structure 228, which again allows for the post installation alignment of the optical fiber end-face in order to maximize the coupling efficiency of the optical train. -
FIG. 1B illustrates a second embodiment of thetunable semiconductor source 200. In this embodiment, thetunable semiconductor source 200 again comprises anoptical bench 205. - Instead of the SLED chip, however, a semiconductor optical amplifier (SOA)
chip 240 is used as the semiconductor gain medium. This chip is also secured to thebench 205 via asubmount 212. - In the illustrated embodiment, the
SOA chip 240 is a reflective SOA. Specifically, the SOA chip'sback facet 242 has a reflective coating in order to function as a back reflector of the laser cavity of thesemiconductor source 200. - Light exiting from the
SOA chip 240 is columnated by afirst lens component 216. It passes through an isolator at 244 to asecond lens component 246 and then to atunable filter 214. - The
filter 214 functions to control which of the longitudinal optical nodes have a net gain in the laser cavity of the laser tunablesemiconductor tunable source 200. - Light exiting from the
filter 214 is collimated by athird lens component 248. - In other embodiments, more or
less isolators 244 are used in the laser cavity. - Specifically, in one example, the tunable laser source has no isolators. Instead, the Fabry Perot
tunable filter 214 is angled with respect to the optical axis of the laser cavity in order to provide tilt isolation. In one implementation, the angling of the Fabry Perot tunable filter is relatively small between one and five degrees. As a result, light that is rejected by the Fabry Perot tunable filter is being out of its passband is not coupled back into the laser cavity in order to be amplified by theSOA gain chip 240. In other example, additional isolators are used to prevent back reflections to the gain chip. - Light passes through an
optical signal regulator 218, as described previously. - The light then passes through a
tap 220 that diverts part of the optical signal to thesource detector 222. The remaining signal is coupled into theoptical fiber 224 via afourth lens component 250. - In one embodiment, the
mirror 272 defining the end of the cavity is located in thefiber 224. In other examples, the mirror is located between theregulator 218 and thefilter 214, thus placing theregulator 218 and thedetector 222 outside of the laser cavity. -
FIG. 1C shows still another example of thesemiconductor tunable source 200. This is also a laser configuration. Instead of a linear cavity, however, it uses aring laser cavity 270. Specifically, a ring cavity is defined bymirrors isolator 244 is located in the ring laser cavity in order to ensure that light travels in only one direction through the ring.Mirror 272 is partially reflective in order to provide a portion of the optical signal circulating in the ring cavity as a tunable output signal. - A
SOA chip 240 provides the gain in thelaser cavity 270.Tunable filter 214 again controls which longitudinal modes of the laser cavity have net gain. - Four
lens components tunable filter 214 and theSOA gain chip 240. Anoptical signal regulator 218, such as a tunable attenuator, is provided at the output of thering cavity 270. Atap 220 is also provided in one embodiment to couple part of the tunable optical signal to thesource detector 222. Alens component 250 couples the remaining light into the optical fiber at 224 that conveys the light to the sample. -
FIG. 2 illustrates the use of thetunable semiconductor sources 200 illustrated inFIGS. 1A through 1C in the construction of a multi semiconductor tunablesource spectroscopy system 100. Specifically, two semiconductor sources 200-1, 200-2 (source 1, source 2) produce tunable signals covering different spectral bands on optical paths or fibers 224-1, 224-2. These are combined to produce an output signal on path orfiber 311 via acombiner 310. In one embodiment, thecombiner 310 is a fiber coupler. In other embodiments, the combiner is implemented on an optical bench using polarization filters or a spectral, dichroic filters, for example. - The output signal on
fiber 311 has a combined scan band that combines the different spectral bands of the two semiconductor sources 200-2, 200-2. In this way, it has a combined scan band that enables scanning over a broader scan band than could be obtained from the sources 200-1, 200-2 individually. - In one embodiment, this output signal on
fiber 311 is conveyed by anoutput signal coupler 312 to anoutput signal detector 314. The remaining output signal is conveyed to asample 10 and then detected by asample detector 12. -
FIG. 3 illustrates the spectral roll-off that is characteristic of the semiconductor sources 200. Overlaid onto the source spectra 410-1, 410-2 are the tunable filters' passbands 412-1 and 412-2 for each of the sources 200-1, 200-2. - Specifically, the semiconductor chips potentially emit in corresponding bands 410-1 and 410-2. The tunable filter passbands 412-1, 412-2 will filter out or block the other regions of the spectrums 410-1, 410-2 in the case of the tunable SLED sources (
FIG. 1A ) or constrain the source to only emit light generated by the optical modes within the passbands 412-1, 412-2 in the case of the tunable laser sources (FIGS. 1B and 1C ). - The sources 200-1, 200-2 cover different spectral bands. For example tunable source 200-1 covers band band-1 stretching from approximately 1330 to 1470 nanometers (nm). In contrast,
source 2 200-2 covers band-2 stretching from approximately 1470 to 1610 nanometers. By combining the output of these two semiconductor sources, a combined scan band stretching from approximately 1330 to 1610 nanometers is achieved. - However, the tunable signals varies in amplitude by approximately 20 decibels dB over this range due to the gain spectrums 410-1 and 410-2 associated with the separate sources 200-1, 200-2. Specifically, these gain bands 410-1, 410-2 suffer substantial roll-off near the limits of the associated semiconductor sources such that it causes changes in the amplitude of the resulting output signal which can impact the analysis of the spectral response of the
sample 10. -
FIG. 4 illustrates a first embodiment control system for compensating for the spectral roll-off of the at least two semiconductor sources, e.g.,SLED sources 210 orSOA sources 240, over the combined scan band provided by the combination of the tunable signals from the two semiconductor sources 200-1, 200-1. - Specifically, source one 200-1 is driven by a source drive circuit of a
control system 400 including asystem detector 314 and acontroller 412. In other embodiments, instead of using thesystem detector 314, thesource detectors 224 are used to sample the magnitude of the signals produced by the sources 200-1, 200-2. - The
source drive circuit 410 controls the current to the diode chips, e.g.,SLED chips 210 of the tunable SLED ofFIG. 1A , orSOA chips 240 of the tunable lasers ofFIG. 1B, 1C . The modulation of the drive current maintains the levels of the respective tunable signals at a fixed level such aslevel 450 ofFIG. 3 . Specifically, near the limits of the scan band, the power or drive current to thechips 210/240 in the sources 200-1, 200-2 is increased in order to achieve a stable level to the output signal across the entire combined scan band. -
FIG. 5 illustrates a second embodiment of thecontrol system 400. Here the control system comprises the attenuators oroptical signal regulators 218 of the sources 200-1, 200-2. Specifically, thesystem detector 314 or alternatively thesource detectors 224 are used to sample the level of the output signal or the tunable signals from the separate sources at 200-1, 200-2. Then, anattenuator drive circuit 414 of thesystem controller 400 is used to drive theattenuators 218 in order to attenuate the separate tunable optical signals in order to achieve astable level 450 to the output signal across the entire combinedscan band 510. -
FIG. 6 shows still another embodiment to the control system. Here thecontrol system 400 comprises thesystem detector 314 or alternatively thesource detectors 224. The signal from the detector is then used to modify the sampled signal detected by thesample detector 12. Specifically, the signal from thesample detector 12 is divided by the signal from theoutput signal detector 314 orsource detectors 224. Specifically, an analog divider circuit incontroller 400 is used in one embodiment. As a result, then the controller 440 generates a compensated sample signal after accounting for the variation in the tunable output signal as detected by thedetector 314. - In another embodiment, this compensation is performed digitally. Specifically, analog to digital converters are used to sample the output from the
output signal detector 314 and thesample detector 312. The controller then operates a software program that performs the compensation. - While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
Claims (15)
Priority Applications (4)
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US10/954,616 US7324569B2 (en) | 2004-09-29 | 2004-09-30 | Method and system for spectral stitching of tunable semiconductor sources |
EP05798334A EP1794555B1 (en) | 2004-09-30 | 2005-09-20 | Method and system for spectral stitching of tunable semiconductor sources |
AT05798334T ATE519099T1 (en) | 2004-09-30 | 2005-09-20 | METHOD AND SYSTEM FOR SPECTRAL MIXING OF TUNABLE SEMICONDUCTOR SOURCES |
PCT/US2005/033714 WO2006039155A1 (en) | 2004-09-30 | 2005-09-20 | Method and system for spectral stitching of tunable semiconductor sources |
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US10/953,046 US7157712B2 (en) | 2004-09-29 | 2004-09-29 | Method and system for noise control in semiconductor spectroscopy system |
US10/954,616 US7324569B2 (en) | 2004-09-29 | 2004-09-30 | Method and system for spectral stitching of tunable semiconductor sources |
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US10/953,046 Continuation-In-Part US7157712B2 (en) | 2004-09-29 | 2004-09-29 | Method and system for noise control in semiconductor spectroscopy system |
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Also Published As
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EP1794555A1 (en) | 2007-06-13 |
EP1794555B1 (en) | 2011-08-03 |
US7324569B2 (en) | 2008-01-29 |
ATE519099T1 (en) | 2011-08-15 |
WO2006039155A1 (en) | 2006-04-13 |
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